In recent years, an optical disc such as a DVD (Digital Versatile Disc) has been used in a wide range of field as a recording medium.
This optical disc has a structure, in which a minute information concave and convex pattern such as grooves to obtain various information signals, for example, an address signal, a tracking signal, and the like and pits as a recording portion for a data information signal is formed on an optically transparent optical disc substrate made of polycarbonate or the like; a reflective film formed of a thin metal film of aluminum or the like is formed thereon; and further, a protective film is formed on the reflective film.
This optical disc is manufactured through a manufacturing process shown in FIGS. 6A to 6J (for example, refer to paragraphs from [0002] to [0006] of the Japanese Laid-open Patent Application No. 2001-195791).
First, a glass substrate 90 having a planarized surface is prepared (FIG. 6A), and a resist layer 91 composed of a photosensitive photo-resist (organic resist) is uniformly formed on the glass substrate 90 to constitute a resist substrate 92 (FIG. 6B).
Then, while relatively scanning recording laser light, for example, in a spiral form from an inner circumference portion to an outer circumference portion of the substrate 90 or from the outer circumference portion to the inner circumference portion thereof on the resist layer 91 of the resist substrate 92, the recording laser light that is ON-OFF controlled corresponding to an information signal pattern is irradiated to form an exposed master 93 to which a pattern exposure, that is, an exposure corresponding to the information concave and convex pattern of an optical disc substrate to be ultimately obtained is performed on the resist layer 91 (FIG. 6C).
Then, the resist layer 91 is developed to obtain a master 94 in which a predetermined concave and convex pattern is formed (FIG. 6D).
Next, a nickel metal plating layer 95 is formed on the plane of the concave and convex pattern of the master 94 by an electroforming method (FIG. 6E). This plating layer 95 is exfoliated from the master 94 and a predetermined processing is performed to obtain a molding stamper 96 onto which the concave and convex pattern of the master 94 is transcribed (FIG. 6F).
The injection molding is carried out using this molding stamper 96 (FIG. 6G) to mold a resin optical disc substrate 97 made of polycarbonate that is a thermoplastic resin material (FIG. 6H).
Subsequently, a reflective film 98 (FIG. 6I) and a protective film 99 made of aluminum alloy are formed on the concave and convex plane of the optical disc substrate 97 to obtain an optical disc 200 (FIG. 6J).
The optical disc manufactured as described above becomes a product after a quality inspection is performed, in which a jitter value (Jitter) is one of the items for measuring the quality. The jitter value shows fluctuation of RF signal in the direction of time axis when signal reproduction is performed and is an important item serving as a quality index of the reproduced signal of an optical disc.
Further, since the jitter is affected by a fluctuation in dimensions of a concave portion (pit) in the concave and convex pattern formed in the optical disc, the value thereof has become a more important control item under the circumstances where the concave and convex pattern becomes minute due to an increase in capacity of optical discs of late.
Accordingly, it becomes important to adjust the shape and dimensions of an exposing spot when making the exposed master which determines the shape and dimensions of the pit. However, since there is a difference in judgment made by individuals whether this exposing spot is good or not good, dispersion occurs in adjusting an exposure focusing at the time of exposure, which has become a cause of dispersion in signal characteristics of the optical disc as an ultimate product.
In addition, since this focus adjustment needs a long optical system and a CCD (Charge Coupled Device) camera for observing reflected light at a confocal point as later described, an optical system structure in an exposing apparatus has become complicated.
Moreover, since the jitter value is obtained from the RF signal pattern at the time of signal reproduction and it is difficult to measure this value from a latent image of the resist layer after exposure, the measurement thereof has only been possible with respect to the optical disc at a stage of an ultimate product (FIG. 6J) after performing the above described manufacturing process.
Therefore, in case that an adjustment of an exposure focusing position was inappropriate, a series of labors and manufacturing time spent until then and also a product became useless.
As described above, when a defect resulting from a manufacturing condition of an exposing process occurs, its loss is unmistakably large.
Further, since it is only possible to take a method of feeding back the measurement result of the jitter value obtained after the above described final process to the manufacturing process, a prompt correction of the manufacturing condition has not been possible, either.
Particularly, with respect to the correction of the manufacturing condition in the exposing process, a long period of time has been required from the time when the relevant lot went through the exposing process until the time when the exposure condition corrected in accordance with feedback information from the final process of the lot can be reflected. Therefore, when a defect of product occurs due to the manufacturing condition of the exposing process, it also takes a long period of time to investigate the cause of defect and furthermore, it takes a huge amount of time before the correction of conditions is reflected in manufacturing, which has also resulted in an obstruction to the whole productivity and decrease in the yield.
Due to the above, in the above described manufacturing process an appropriate manufacturing condition of each process is set such that the fluctuation in dimensions of an information concave and convex pattern of the optical disc, particularly of the pit, can be controlled as much as possible and the jitter value is managed to fall within a certain range.
Particularly, the above described exposing process is a process which greatly affects the formation of a pit, and especially among other things, a strict control to maintain the distance between an objective lens of the exposing apparatus and the surface of the resist layer of the resist substrate (hereinafter, referred to as an exposure focusing position) is required, because exposure must be performed such that the recording laser light is focused on the surface of the resist layer of the resist substrate.
Conventionally, the adjustment of the focus position in the exposing process is performed using a method in which a position (height) of the resist substrate is fixed; light reflected from the resist substrate is visually observed at such a position as a focal point of the objective lens becomes a confocal point; and the height position of the objective lens from the surface of the resist layer of the resist substrate is adjusted by operating a focus actuator which performs a focus adjustment such that its spot shape can become the best.